1. Field of the Invention
The present invention relates generally to a method of separating two polarization states of an input optical signal, and more particularly, but not by way of limitation, a passive polarization sorter for separating the Transverse Electric (TE) polarization state and the Transverse Magnetic (TM) polarization state of the input optical signal into at least two geometrically separated outputs.
2. Brief Description of the Related Art
Light is a vector field that has two primary and orthogonal polarization states or vector directions. Generally, the polarization states are referred to as the S and P polarizations in free space optics, or the TE (Transverse Electric) and TM (Transverse Magnetic) modes of optical waveguides. The performance of optical waveguides and optical devices is often sensitive to the polarization state. That is, the response of the device changes as the polarization state changes. This is particularly pronounced in integrated optical waveguides that are fabricated on dielectric substrates.
Typically, it is desirable to have optical components that are insensitive to the input state of polarization. In fiber optic telecommunications, the polarization state of an optical signal that has traveled down any length of fiber is unknown, random, and time varying (due to perturbations in the environment). Great care is often taken in the design and fabrication of optical components so that they behave in a polarization insensitive manner. Despite this effort, many devices remain polarization sensitive to some degree, and this affects ultimate performance, yield, and cost.
There are some special applications where the two polarization states of an input optical signal needs to be spatially split so each can be manipulated independently, such as, for example, PMD (Polarization Mode Dispersion) compensators, where the dispersion of the signal on the two states needs to be equalized. In applications where the polarizations need to be split, the extinction ratio, which is the ratio of wanted to unwanted polarization in either of the two branches, must be high.
Generally, another way to handle polarization in a device that is required to behave as if it were polarization insensitive is to split the input polarization into two branches having orthogonal states, process each branch independently with devices that are optimized for each polarization respectively, and then recombine the processed signals together. This scheme is referred to as “polarization diversity”. Each branch can be specifically optimized for its respective polarization, giving the best performance without otherwise having to compromise the ability to give adequate performance over two polarization states simultaneously.
Traditionally, optical components have been quite large, and polarization diversity schemes have not been popular because of the added size and cost associated with packaging twice the componentry plus the splitters. Prospects for polarization diversity improve for integrated optics fabricated on substrates, where the objective is to shrink the size of components and to integrate various functionalities on a common die or chip, similar in concept to integrated electronic circuits (ICs). Polarization splitters and two sets of components are fabricated all at once. Future integrated optical components are miniaturized by the use of high-index contrast waveguides. High-index waveguides themselves are more susceptible to polarization sensitivity. Polarization diversity may be the only path forward for these future high-index contrast components.
Polarization sorters, also called polarization beam splitters or simply polarization splitters, are important building block elements in integrated optics and planar lightwave circuits. In polarization diverse optical circuits where the polarization states of an input optical signal are separated and processed independently, polarization sorters are essential.
A polarization sorter separates the two orthogonal polarization states of an input optical signal into two geometrically separated outputs. The arbitrary input signal is composed of two principle states of polarization. In planar lightwave circuits and integrated optics, these states are commonly referred to as the TE polarization state, and the TM polarization state. The TE state is characterized as that state where the electric field is predominantly polarized parallel to the optical circuit substrate, while the TM state is characterized as that state where the magnetic field is predominantly polarized parallel to the optical circuit substrate (Dietrich Marcuse, “Theory of dielectric optical waveguides”, New York, Academic 1974). For an arbitrary input signal, the relative amounts of power in the TE and TM states are both arbitrary and can also be time-varying.
Therefore, there is a need for an effective and efficient waveguide structure for separating the polarization of an optical signal.